As an inspection apparatus for a semiconductor device and a wafer, an apparatus employing a charged particle beam apparatus that irradiates a charged particle beam on a sample held in a vacuum sample chamber and detects a signal secondarily generated from the sample to obtain an image is widely used. When a defect is present in a sample to be inspected that is an inspection target of the inspection apparatus, an amount or a state of generated secondary electrons changes. The inspection apparatus to which a charged particle beam is applied acquires an image of a region including a defective place, compares the image with an image of a normal place, and inspects the image to thereby specify the defective place. An image contrast due to the secondary electron signal is sensitive to the surface potential of the sample. Therefore, a charged particle beam inspection apparatus can detect defects related to electrical characteristics present in a circuit pattern of a semiconductor wafer, which cannot be detected by an optical inspection apparatus, for example, defects such as short circuit, disconnection, and opening failure of a hole (hereinafter, electrical defects).
Since the defect inspection is a detecting method by comparison and inspection, in order to enable detection of a defect, it is necessary that there is difference in an image contrast (hereinafter referred to as potential contrast) between the defect image and the image of the normal place. Therefore, in order to detect an electrical defect with high sensitivity, it is necessary to charge a sample surface before observation or during observation to provide a difference in surface potential between a defective place and a normal place.
It is known that charging of a wafer progresses as a kind of a relaxation phenomenon. When the wafer is charged, a positive or negative voltage is applied to an electrode, which is referred to as a charging control electrode, set immediately above the wafer and a charged particle beam is irradiated on the wafer in that state. Secondary electrons generated from the wafer are absorbed in the control electrode when the voltage applied to the control electrode is positive and are returned to the wafer when the voltage applied to the control electrode is negative. Therefore, a shortage state of electrons or an excess state of electrons is formed on the wafer according to the polarity of the applied voltage to the control electrode. As a result, it is possible to charge the wafer to a desired positive or negative value according to the polarity of the applied voltage to the charging control electrode. Irrespective of to which polarity the wafer is charged, the wafer is charged in a direction in which a wafer surface voltage approaches the voltage of the charging control electrode. Therefore, when the wafer is charge to a voltage generally equal to the control voltage, incident electrons and electrons generated from the wafer are balanced and the surface voltage of the wafer is stabilized.
Various methods have been developed concerning a charging method of a sample to be inspected. However, basically, a charged state is formed on the basis of the principle explained above. For example, JP Patent Publication (Kokai) No. 2006-234789 (Patent Literature 1) discloses an invention for preliminarily irradiating an electron beam on a sample to be inspected from an electron source (a flood gun) different from a beam for inspection and charging the sample before irradiation of the beam for inspection.
Defects present in a circuit pattern on a semiconductor wafer often concentrate in a range of several tens millimeters from the outermost circumference of a wafer (hereinafter referred to as wafer outer circumferential portion) for convenience of a manufacturing process. Therefore, it is necessary to highly accurately inspect the wafer outer circumferential portion. However, a wafer end is a boundary surface between the wafer and a vacuum (or the atmosphere) and an electric field (hereinafter, peripheral electric field) is formed on the boundary surface. A charged particle beam irradiated on a sample is bent and distortion and positional deviation of an image occurs because of distortion of the peripheral electric field.
In FIG. 19, a peripheral electric field formation mechanism of a wafer end that occurs when a wafer is charged by the method of using the flood gun is schematically shown. FIG. 19(a) corresponds to positive charging and FIG. 19(b) corresponds to negative charging. In a charged particle beam microscope, for protection of the wafer end, usually, the diameter of a wafer holder 1902 is designed larger than the diameter of the wafer. Therefore, a part of an irradiated electron beam 1901 irradiated to charge the wafer is also irradiated on a wafer holder circumferential edge 1907 on the outer side of the wafer. However, since the wafer holder is usually made of metal, if a potential difference occurs, an electric current flows to the wafer holder and potential fluctuation does not occur in the wafer holder circumferential edge.
When a voltage 1906 positive with respect to a wafer 1903 is applied to a control electrode 1904 (FIG. 19(a)), secondary electrons 1905 are generated from the wafer and the wafer is charged in positive polarity. On the other hand, since the potential does not change in the wafer holder circumferential edge 1907 on the outer side of the wafer, the wafer holder circumferential edge 1907 relatively has negative polarity with respect to the wafer charged in the positive polarity. As a result, a part of secondary electrons 1908 generated from the wafer holder are supplied to a wafer outer circumferential portion 1909 and the positive charging of the outer circumferential portion 1909 is weakened.
When a voltage 1910 negative with respect to the wafer 1903 is applied to the control electrode 1904 (FIG. 19(b)), the secondary electrons 1905 generated from the wafer are returned to the wafer itself and the wafer is charged in negative polarity. On the other hand, since the potential does not change in the wafer holder circumferential edge 1907, the wafer holder circumferential edge 1907 relatively has positive potential with respect to the wafer charged in the negative polarity. As a result, a part of the electrons returned to the wafer outer circumferential portion 1909 or secondary electrons 1911 generated in the wafer outer circumferential portion are supplied to the wafer holder circumferential edge 1909 and the negative charging of the wafer circumferential portion 1909 is weakened. From these results, the charging potential of the wafer outer circumferential portion is different from the charging potential of the wafer center portion.
As a method of suppressing such an effect of the peripheral electric field, JP Patent Publication (Kokai) No. 2004-235149 (Patent Literature 2) disclosed a technique for arranging a ring-like conductive member on the outer side of a wafer holder that holds a wafer (hereinafter, wafer holder circumferential edge), applying a DC voltage to the conductive member on which an inspection beam is being irradiated, and reducing the influence of electric field distortion on an electron beam made incident on the wafer outer circumferential portion. The invention disclosed in this literature has an object of, rather than charging control, relaxing, with potential applied to the ring-like conductive member, the gradient of a peripheral electric field formed at a wafer end and enabling the inspection beam to accurately reach a sample surface.